The objective of the study is to compare the efficacy, safety, and tolerance of a 50mg dose of a new antiprogestin, CDB 2914 with a 0.75mg dose of levonorgestrel as emergency contraception. This is a multicenter, randomized, double-blind phase II study intended to test a one dose regimen for emergency contraception against the current standard regimen. Subjects have two visits, enrollment and follow-up. If the subject has not had menses by her scheduled follow-up visit, she will be followed until she has her period, or it is determined that she is pregnant.